Summary The pilot instructor and his student were making touch-and-go landings on an Aerocruiser advanced ultralight. Shortly after take-off from the private runway of cole de pilotage de Lavaltrie, Quebec, at a height of approximately 300 feet above ground level, one of the propeller blades separated. The pilot instructor shut down the engine and landed the ultralight in an unprepared field. On the ground, the aircraft struck a tree and flipped over. The two occupants of the aircraft sustained serious injuries in the ground impact. Ce rapport est galement disponible en franais. Other Factual Information The pilot was qualified for the flight. The aircraft had been rented from a company that operated and maintained it. The aircraft had logged approximately 437 flying hours since new. It was equipped with a Rotax 912 four-stroke engine. A two-blade wooden propeller with a diameter of 72 inches (GSC International Inc.) was mounted on the engine. The propeller had been installed by the operator and had logged just over 300 flying hours since it was purchased in 1994. On several occasions, the operator of the aircraft had to adjust the pitch of one of the propeller blades. To rectify this problem, the operator had machined the propeller hub down 0.010 inch in order to apply greater torque at the blade roots. The blade was recovered, and an examination of the propeller was conducted at the TSB Engineering Branch Laboratory. The marks made by the bolts running through the two blades show that the blades were set at a medium pitch before the accident and that the pitch had not changed. The broken section of the propeller indicated that the failure was progressive. Two anomalies were found on the broken blade: there was a crack in the wood of the root, and a black adhesive tape covered the circumference of the urethane capsule at the blade root, increasing the adjustment of the capsule in the propeller hub. The hub measurements were within the manufacturer's specifications. At the root of the second blade, there was a crack at the same location as in the broken blade. No other damage was observed on the blades. According to the engineering report, these cracks could have been caused by contact of the blades with a soft object, such as water or snow, or by insufficient or excessive torquing of each blade root. The propeller load calculations corresponded to those of the manufacturer, and fabrication of the propeller complied with the required and applicable standards. The propeller manufacturer reported that there had been four similar occurrences out of the 10 000 blades in service. These occurrences were attributed to engine overspeed beyond the operating range specified by the manufacturer, impact with an object, or improper installation of the blades in the hub.